Liquid-cooled electric motor generator units are used in, among other things, the field of commercial agricultural vehicles and are used there to implement infinitely switchable gear assemblies or for the electric supply of additional units of the commercial agricultural vehicle by means of a crankshaft generator or an external auxiliary generator. The motor generator units, designed mostly as synchronous motor generators, have a stator with a stator lamination package in addition to associated stator windings and a permanently magnetized motor that is mounted such that it can rotate within the stator. In order to remove the heat loss from the stator windings and caused during operation, the synchronous motor generator has an inlet which cooling liquid under pressure can be conducted from the outside through a magnetic air gap running between the stator and rotor. The cooling liquid found in the air gap results in the appearance of efficiency-reducing shearing and friction forces between the stator and rotor, wherein they increase with the pressure and volume flow of the supplied cooling liquid. The cooling liquid is a hydraulic or gear oil branched off from a central provisioning system of the commercial agricultural vehicle or the external auxiliary generator whose pressure or volume flow is fixed by the system, so that a suitable compromise can be found between the required cooling performance, on the one hand, and the efficiency of the synchronous motor generator, on the other hand.